Extracellular ATP regulates exocytosis by inhibiting multiple Ca²⁺ channel types in bovine chromaffin cells

Abstract

Feedback modulation of voltage-dependent Ca²⁺ channels by ATP is a well documented phenomenon in bovine chromaffin cells. However, its influence in the control of hormone release is at present poorly understood. By using combined patch-clamp and fura-2 fluorescence measurements we provide evidence that the three Ca²⁺ channel types (L, N and P/Q) expressed in bovine chromaffin cells are inhibited by ATP (30 µM), and that their involvement in the secretory response, as assayed by capacitance measurements, is roughly proportional to their contribution to the whole-cell Ca²⁺ current (I _Ca) both in the absence and presence of ATP. ATP did not modify the capacitance increase observed in cells dialyzed with Ca²⁺-EGTA buffers (1.5 µM free Ca²⁺), thus excluding a direct effect of ATP on the secretory machinery. Voltage predepolarizations or long chemical (2 s, 70 mM KCl) depolarizations attenuate the effect of ATP on exocytosis by partially relieving the inhibition of I _Ca. Likewise, a strong stimulation that depletes the readily releasable pool of vesicles prevents an inhibitory effect of ATP on the secretory response. While these results lend support to the hypothesis of autocrine modulation of exocytosis by endogenously released ATP acting on P2y-purinoceptors to inhibit I _Ca, feedback regulation of the rate of release will be a complex function of the occupancy of those receptors and of the electrical and secretory activity of the cell.